The present invention relates to a process for heat treatment of a radioactive substance by microwave power and more particularly, to a heat treatment process for the production of oxide powders suitable for the manufacture of nuclear fuel pellets.
Typical conventional methods of producing oxide powders of uranium, thorium, plutonium or the like substances include a direct denitration method, a sol-gel method, a precipitation method and so forth. These methods have, however, advantages and disadvantages, and are not satisfactory.
The direct denitration method comprises heating nitrate solutions of these substances to convert the nitrate solutions to oxide powders and, according to the type of heating, this method further employs a fluidized bed or heater. In the direct denitration method using a fluidized bed, there are produced hollow particles which are unsuitable for raw materials for fuel pellets of high density. This method has further disadvantages in that a large amount of gas is discharged due to the use of fluidizing gas, that a large-sized apparatus for processing the discharging gas is required, and that additional oxide powders are necessitated as a fluidizing medium and as seeds. On the other hand, according to the direct denitration method using a heater wherein heating is directly carried out by the heater, there is a tendency to heat the particle from the side close to the heater, so that an oxide layer is produced at that side of the particle. Such oxide layer serves as an insulating material and binders further heating of the central portion of the particle. When the heating is further continued to heat the particle up to the central portion, the side of the particle close to the heater is highly heated, resulting in the production of powders of a low activity due to a poor sinterability.
The sol-gel method comprises, in the case where uranium oxide, for example, is produced, adding hexamethylenetetramine to a uranyl nitrate solution, dripping the thus obtained solution into paraffin at 95.degree. C. to make sol and gel, and drying it to produce oxide powder. This method presents problems in that it is difficult to define and control conditions for producing oxide powder having a desired particle size, and that fuel pellets made of the thus obtained powder provide a larger degree of swelling.
The precipitation method consists of preparing a nitrate solution of uranium, thorium, plutonium or the like substances, adding ammonia to the solution so as to cause precipitation of hydroxides, subjecting the precipitate to solid-liquid separation, drying the separated solid and effecting roast-reduction of the dried solid. This precipitation method is most preferred because it can provide powders of a high activity due to good sinterability, as well as a smaller swelling of the pellet. This method, however, requires a large number of steps such as solid-liquid separation, drying, roast-reduction and so on as stated before, resulting inevitably in enlargement and complication of the production equipment, as well as an increase of the dosage of radioactive rays to which workers are subjected. If remote control and automation of the process are adopted for reducing the dosage to the workers, the equipment is inconveniently rendered further complicated. During maintenance of such highly complicated equipment, the chance of increased dosage will be increased.
Namely, since the substance to be treated is radioactive, the process must be executed under a specific management, and the substance must be handled in a cell or globe box of gas-tight construction having a specific evacuation system. The simplification and improved durability of the processing equipment are essential particularly in this case, because the control and protective maintenance of the equipment are made indirectly. The reduced number of process units is preferred also from the view points of decrease of dosage to workers and reduction of generation of radioactive byproducts which are to be disposed.